Running cmake resulted in an error message about SDL2_INCLUDE_DIR, so obviously the find_library call had succeeded but the variable value wasn't set regardless.

$ cmake .
-- Configuring done
CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
SDL2_INCLUDE_DIR
...

Adding Debug Output

So I added some logging messages about SDL2_INCLUDE_DIR and SDL2_LIBRARY as well.